eGFR or creatinine clearance: which should you use?
Use eGFR for most drugs and creatinine clearance for the cases where precision matters. The eGFR reported by laboratories is standardised to a body surface area of 1.73 square metres, so it can over- or under-estimate the function of a small or large patient.
The BNF recommends calculating creatinine clearance with the Cockcroft-Gault equation, using the weight the BNF specifies (usually actual body weight, but ideal or adjusted weight in obesity), in these situations:
- Drugs with a narrow therapeutic index or that are nephrotoxic.
- Older people, and patients at extremes of body weight.
- DOACs, whose licensed dosing is written in terms of creatinine clearance.
- Any drug where the monograph states creatinine clearance rather than eGFR.
Creatinine itself is a poor guide in the frail and in low muscle mass, because a normal creatinine can hide a low clearance. Use the creatinine clearance, and check how the BNF band for that drug is expressed.
Which drugs are nephrotoxic or harm the kidney?
Nephrotoxic drugs either damage the kidney directly or reduce its perfusion, and they accumulate in patients with a low GFR.
- NSAIDs: reduce renal blood flow, especially with an ACE inhibitor or ARB plus a diuretic.
- ACE inhibitors, ARBs and diuretics: haemodynamic effects in volume depletion or renal artery stenosis.
- Aminoglycosides, vancomycin and amphotericin.
- Ciclosporin and tacrolimus.
- Intravenous contrast agents.
- Lithium: also causes nephrogenic diabetes insipidus.
- Trimethoprim: raises serum creatinine without changing true GFR and can raise potassium.
In acute kidney injury, stop or hold nephrotoxic and renally cleared drugs, review all prescriptions, and restart them according to recovery.
Drugs to avoid or reduce in renal impairment
| Drug or class | Problem | What to do |
|---|---|---|
| NSAIDs | Reduce renal perfusion; AKI | Avoid in significant impairment; avoid in AKI |
| Metformin | Lactic acidosis risk | Avoid if eGFR below 30; review dose if eGFR below 45; stop in AKI |
| DOACs | Accumulate; bleeding | Dose or avoid by creatinine clearance; dabigatran most renally cleared |
| LMWH | Accumulates | Reduce dose or use unfractionated heparin in severe impairment |
| Gabapentin and pregabalin | Excreted unchanged; sedation, confusion | Reduce dose by renal function |
| Morphine | Active metabolites accumulate | Avoid or reduce; prefer opioids with less renal clearance per local palliative guidance |
| Digoxin | Renally excreted; toxicity | Lower dose and levels; correct potassium |
| Lithium | Renally excreted; toxicity | Avoid in severe impairment; specialist-managed |
| Nitrofurantoin | Ineffective at low GFR; toxicity | Avoid below the BNF eGFR threshold |
| Trimethoprim | Raises creatinine and potassium | Reduce dose in impairment; check potassium |
What are the sick-day rules?
Sick-day rules tell patients to pause certain drugs during dehydrating illness such as vomiting, diarrhoea or fever. The aim is to prevent acute kidney injury, hyperkalaemia and lactic acidosis.
The drugs to pause are often remembered as SADMANS:
- Sulfonylureas (hypoglycaemia).
- ACE inhibitors.
- Diuretics.
- Metformin.
- ARBs.
- NSAIDs.
- SGLT2 inhibitors (ketoacidosis and volume depletion).
Restart them after the patient has been eating and drinking normally for 24 to 48 hours. Patients on regular steroids need a different rule, because stopping them is dangerous, so they follow steroid sick-day guidance.
Where do you find renal dosing in the BNF?
Each drug monograph has a Renal impairment section that gives dose adjustments, usually by eGFR or creatinine clearance bands. Read the unit and the band before applying the adjustment.
- Open the monograph and go to Renal impairment.
- The guidance on prescribing in renal impairment in the BNF explains eGFR, creatinine clearance and the Cockcroft-Gault equation.
- Check the Indications and dose section first, because adjustments differ between indications.
- Look at Cautions and Contra-indications as well as the renal section.
In the exam, pick the number the case gives, convert it to the unit the monograph uses, and then choose the band.
Check yourself
- When should creatinine clearance be used instead of eGFR?
- Why can a normal creatinine mislead in an older, thin patient?
- What is the BNF threshold for stopping metformin, and what about review?
- Which drug in the triple whammy is a diuretic, and what are the other two?
- Why is morphine a poor choice in severe renal impairment?
- What does SADMANS stand for?
- When do patients restart sick-day drugs?
- Where is renal dosing in a BNF monograph?
Questions people ask
Should I use eGFR or creatinine clearance for dosing?
Use eGFR for most drugs. Use Cockcroft-Gault creatinine clearance for DOACs, narrow-therapeutic-index and nephrotoxic drugs, older patients and extremes of body weight, as the BNF advises.
At what eGFR is metformin avoided?
The BNF advises avoiding metformin if eGFR is below 30 and reviewing the dose if it is below 45. Stop it during acute kidney injury.
What are sick-day rules?
Temporary pausing of drugs such as ACE inhibitors, ARBs, diuretics, metformin, NSAIDs, sulfonylureas and SGLT2 inhibitors during dehydrating illness, restarting after 24 to 48 hours of normal eating and drinking.
Why avoid morphine in renal failure?
Its active metabolites accumulate and cause prolonged sedation and respiratory depression. Other opioids with less renal clearance are usually preferred.
Why does trimethoprim raise creatinine?
It blocks tubular secretion of creatinine, so serum creatinine rises without a true fall in GFR. It can also raise potassium.
Related revision notes
Sources: BNF; BNF guidance on prescribing in renal impairment; NICE guidance on acute kidney injury and chronic kidney disease; MHRA Drug Safety Update. These notes are for exam revision and are not patient-specific advice; in the exam and in practice, the BNF is the authority.
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